The present invention relates to a novel planar avalanche diode construction and more particularly to such diodes having a breakdown voltage between 4 and 8 volts.
It is pointed out that the breakdown of diodes is dependent on two separate physical phenomena. In the range of low breakdown voltages (below 4 V) a Zener effect occurs, i.e. a transition of the bonding electrons in the conduction band under the effect of a very high electrical field. In the range of higher voltages (above 8 V) there is an avalanche effect by carrier multiplication in the space charge zone. Thus, from the theoretical standpoint, a distinction should be made between a Zener diode and an avalanche diode. In practice, these diodes are either called Zener diodes or avalanche diodes, independently of the physical phenomenon involved. For example, the breakdown voltage is generally designated by the reference V.sub.Z, which normally refers to the Zener effect, even in the case where there is in fact an avalanche effect.
In FIG. 1, dotted lines are used for a characteristic curve 10 of a Zener diode with a threshold voltage for example below 4 V, whilst continuous lines indicate a characteristic curve 11 of an avalanche diode with a breakdown voltage for example above 8 V. In general terms, it should be noted that the characteristic curve of a diode operating by the avalanche effect has a very marked bend and a low dynamic impedance R.sub.ZK in the vicinity of the bend. However, with a diode functioning with the Zener effect, for reasons inherent in said effect there is a relatively rounded bend, i.e. a higher dynamic impedance.
Greater interest is attached here to Zener diodes whose breakdown voltage is between 4 and 8 V. In this range, the physical effect involved is partly a Zener effect and partly an avalanche effect and, as a function of the construction methods used, breakdown characteristics with a more or less rounded bend are obtained.
FIGS. 2A and 2B show two conventional planar Zener diode constructions according to the prior art. FIG. 2A shows a planar Zener diode in which a P diffusion 1 is formed in a N substrate 2 by means of a silicon dioxide mask 3 providing a window through which diffusion takes place. By using this construction, Zener diodes with a relatively pronounced bend are obtained in this voltage range. However, these diodes have the disadvantage that the characteristics vary as a function of the duration of use, i.e. as a function of time the bend becomes rounder and the breakdown voltage can vary, whilst the inverse current increases. To obviate this, the prior art uses the construction shown in FIG. 2B in which the P diffusion 1 shown in FIG. 2A is surrounded by a zone 4 of deeper diffusion P. This construction is normally called the guard ring construction. It should be noted that for guard ring diodes as shown in FIG. 2B, reliability is better than in the case of the diodes shown in FIG. 2A, i.e. the initial characteristics are maintained as a function of time. However, these guard ring diodes have a breakdown characteristic with a much rounder bend than diodes without a guard ring, as shown in FIG. 2A.
Thus, in practice, it is necessary to make a choice between a diode with good initial characteristics, but poor reliability and a diode with good reliability, but relatively unsatisfactory breakdown characteristics.